1. Field of the Invention
The present invention relates to inter-cell separators in batteries.
2. Description of the Prior Art
Batteries that are to be used in space need to be reliable, maintenance free, have high weight and volume energy densities, and be able to operate without the aid of gravity. Generally, aqueous alkaline batteries have been used in space applications because of their high energy densities and the fact that the electrolyte concentration does not vary with the state-of-charge. Specifically, nickel-cadmium or nickel-hydrogen batteries have been used in this situation. See U.S. Pat. Nos. 3,877,985 and 4,567,119, for example.
In the nickel cadmium system, excess oxygen is evolved at the nickel electrode and diffuses through a porous unflooded separator to react at the cadmium electrode to form cadmium hydroxide, which is subsequently reduced to cadmium metal. If the capacity of the nickel electrodes is slightly less than the capacity of the cadmium electrodes, the products of overcharging a given cell are usually recombined into the reactants normally present in the cell.
In another example, the nickel hydrogen system is pressurized with hydrogen. Any oxygen produced at the nickel electrodes during charging will diffuse across the porous cell separator and be recombined with excess hydrogen to form water at the catalyzed hydrogen electrode. In both cases, oxygen may diffuse across the porous separator to react with the negative electrode material, thereby eliminating gas accumulation and restoring lost electrolyte. See U.S. Pat. Nos. 3,817,771 and 4,087,893.
When higher power and energy densities are required, silver electrodes have been used to replace the nickel electrodes. However, silver oxide has a higher solubility in the electrolyte than nickel oxide. Thus, a much less porous diffusion barrier must be included in the cell separator in order to avoid migration of silver to the negative electrode. However, this also limits gas diffusion and subsequent recombination which in turn limits its application in space. Furthermore, high rate cells usually have thinner electrodes and electrolyte gaps, which further exacerbates the migration and recombination problems.
For high rate batteries, it is preferred that such cells are usually connected in a series in a bipolar stack configuration. In such an array, if gas displaces the electrolyte in a single cell, the resistance of the entire battery is increased and the whole device may become electrically open with the full battery voltage appearing across that cell, resulting in a reduction in dependability.
In large, thin, high rate cells, electrolyte circulation is not practical due to the complexity, propensity to leak and the reduction in cell performance due to shunt currents, which are parasitic currents between cells through the electrolyte circulation channels. Therefore, external recombination of cell gases may be precluded because in space there is no means for gas disengagement from the electrolyte. Gas recombination must take place over the entire cell area, as is the case where gas diffusion occurs through intercell separators.
Many types of intracell gas recombination elements and gas permeable separators are known, some of which include polytetrafluoroethylene-carbon combinations. See for example, U.S. Pat. Nos. 4,374,907; 4,339,325; and 3,930,890. However, these are not useful for space applications due to limitations such as requiring gravity to function and/or the need to resist silver ion migration while providing reactant gas recombination.
In spite of these disclosures there remains a need for a bipolar battery, especially a silver oxide iron battery, that can be used in space and be reliable, maintenance free, have high energy densities and provide gas recombination without destroying the cell.